Video watermarks are useful for a variety of purposes in the domain of pay television, such as for allowing a watermark detector to identify the creator of a video, the operator broadcasting the video, or even the identity of the subscriber receiving the video. The latter is known as forensic watermarking.
Embedding forensic watermarks at the video headend has advantages over client-side embedding for computational power and security reasons. Such embedding involves the video headend typically broadcasting multiple versions of sections of the video, each of the versions being visually slightly different from each other.
H.264 and H.265 video encoders typically use the Context-based Adaptive Binary Arithmetic Coding (CABAC) entropy encoding system for the entropy coding layer of macroblock-level syntax elements. CABAC is a highly stateful, very adaptive entropy layer encoding system that can encode similar streams of syntax elements into significantly differing bitstreams because of how CABAC adapts its context models as it processes the input streams. In other words, even if two frames of video differ in just a single syntax element (such as a prediction mode of one macroblock, a coefficient in the residual of a certain macroblock etc.), the resulting post-CABAC bitstream typically completely differs, from the point in which the syntax elements differ until the CABAC engine is reset, according to the video encoding standard.